A photovoltaic electricity generating system comprises strings, in which multiple electricity generating modules are connected in series, and senses electrical output (energy or current) for each string. The electrical output of an electricity generating module gradually decreases because of deterioration over time even under the same sunshine conditions. It is also possible that electrical output suddenly decreases because of a fault such as deterioration of product quality or physical damage. The output of such a faulty electricity generating module falls to almost zero, and the module no longer contributes to electricity generation.
To repair or exchange an electricity generating module whose electrical output has suddenly decreased as soon as possible, a fault detection apparatus is necessary. For example, if sensors (such as ammeters) capable of measuring the electrical outputs of all electricity generating modules are provided, it is possible to recognize a faulty electricity generating module if its sensor indicates zero when the level of sunshine is high.
However, it is difficult to provide electrical output sensors for all electricity generating modules because of the manufacturing cost. A conventional system recognizes that a string contains a faulty module by comparing the electrical outputs of strings. For example, the system calculates the average electrical output per string, and determines there is a fault if the actual electrical output of a string is below average by 20%.